The Cavaquinho is a small, usually 4 stringed, 4 course chordophone that originated around the town of Braga in northern Portugal. I won't go into the dim and distant history of its origins, (check out the Museu Cavaquinho Website if you want this), just say that it has been around in the region for hundreds of years. It is one of a large number of Portuguese folk chordophone and one of the, if not the smallest of them with a scale length of around 350mm (13in). It proved quite popular in Portuguese culture, spreading from its northern roots across Portugal, and then across the Portuguese empire becoming the Braguinha, the Cavaco, the Cak and Cuk, and eventually breaking out to become the Ukulele, (with a bit of help from the Rajao). Traditionally the oldest ones, (often called Minho Cavaquinhos), from the Braga region, (which use to be called the Minho region until 1976), has a figure 8 body shape, saddleless bridge attached to the lower bout but a small floating saddle just after this, and the fairly wide, (Ukulele like), neck flush to the body with 11 or 12 frets but beyond this there are a number of variations in style. Sometimes there is a golpeador but not always, sometimes the is a round central sound hole but not always, sometimes they are highly decorated but again not always and sometime they come with an archaic set of tuning pegs the Portuguese call "English(?)" but not always, finally they are usually steel strung, but not always! 

The major Portuguese variant to the Minho Cavaquinhos comes from the transition south and are sometimes called Lisboa Cavaquinhos. These are usually a lot thinner in the body shape, (but still figure 8), have a raised fretboard that extends to the 17th fret, (but still meets the body at the 12th), and are far more likely to have a round central sound hole, no golpeador and a bridge with a saddle, (but not always, particularly in transitional instruments). The tuning for both variants is subject to change too, it is usually an open chord but can be D~G~B~D (where the high D is an octave above the low D) or D~D~G~B (where the two D's are the same note), sometimes the root chord changes and sometimes they have a more Guitar or Mandolin tuning. 

There are Banjo Cavaquinhos too, but for some reason these are more commonly 8 string 4 course instruments? (I have seen 8 string Cavaquinhos but they are not common and I have seen 4 string Banjo Cavaquinhos too). These always have an enclosed drum and a smallish bracketless head that has a metal grill around it to project the sound through. For some reason they are often called Banjolims too? (with an M not and N; the Portuguese do make Banjo Mandolins too but these have a bigger head and a narrower neck)

Before the spread to Hawaii and the Ukulele, there is some evidence of them outside of the Portuguese empire in Germany. Cavaquinho like instruments appear in a few 19th c. makers/distributors catalogues, though whether the instruments were made in Germany or imported for Portugal for distribution I don't know, (it is also possible they are single course Mandolinettos either locally produced or from Italy)? In the very early 20th c as the Ukulele started to enter world consciousness, these instruments are rechristened Hawaiian Mandolins before eventually becoming Ukuleles themselves.

Under the recent influence of the Ukulele, Cavaquinho design has become a lot freer with things like cutaways and round bodies being made. Also there are some smaller Mini equivalents being produced now, (historically there were no scale variations), but all the ones I have seen have been referred to a piccolo. 

One final note about names - when you look at a lot of "academic" texts on Cavaquinhos you will see them all trot out a long list of pseudonyms, (Wikipedia for example says "It is also called machimbo, machim, machete (in the Portuguese Atlantic islands), manchete or marchete, braguinha or braguinho, or cavaco and ukulele (in Hawaii)"  Well ignoring the Ukulele as it is a different instrument altogether, Braguinha is what is actually used on the Madeiran archipelago and Cavaco is used in Brazil and Cape Verde to describe their fairly different version. Beyond that I have never seen any of these terms used to describe any actual instruments, its just a list copied by one lazy "scholar" from another, (and yes I am aware of some people quoting from 17th c. texts and the like, what I mean here is these names are no longer used and haven't been for a at least 150 years)